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SECONDARY CARBIDE DISSOLUTION and COARSENING in 13% Cr MARTENSITIC STAINLESS STEEL DURING AUSTENITIZING

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SECONDARY CARBIDE DISSOLUTION and COARSENING in 13% Cr MARTENSITIC STAINLESS STEEL DURING AUSTENITIZING SECONDARY CARBIDE DISSOLUTION AND COARSENING IN 13% Cr MARTENSITIC STAINLESS STEEL DURING AUSTENITIZING A Dissertation presented by Ming Laura Xu to The Department of Mechanical and Industrial Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the field of Materials Science and Engineering Northeastern University Boston, Massachusetts April 2012 1 ABSTRACT Cutting blades and knives in various forms are manufactured from martensitic stainless steel strips. The manufacturing process of these cutting knives comprises a hardening heat treatment, cutting edge formation, and shaping into product dimensions. In a production environment, the hardening heat treatment is typically carried out continuously using an in-line heat treatment system. Such a heat-treatment process enables high production speed and efficient through-put. However, a high speed in-line heat-treatment process is very sensitive to raw material variations. Such variations may arise from differences among the manufacturing processes employed at raw material suppliers as well as shipment to shipment quality variations from a supplier. Some of these variations can be very subtle and might not have been fully understood by conventional material characterization techniques. The subtle material variations could cause differences in the response of the materials to the heat treatment, thereby potentially impacting the downstream manufacturability as well as the performance of the finished products. In addition, with the increasing demand for higher through-put production, optimizing the process parameters has become even more crucial. Therefore, the purposes of this work were to study the physical metallurgy of the hardening process and ultimately develop a simulation model to predict the kinetics of secondary carbide dissolution and coarsening during the austenitizing treatment of martensitic steel. The steel studied in this work mainly contains 0.7 wt. % C and 13% of Cr, which is a non- AISI standard martensitic stainless steel. Detailed material characterization was carried out using 2 advanced quantitative metallographic techniques to characterize the subtle materials variations. The secondary carbide size distributions before and after the hardening process were characterized by scanning electron microscopy (SEM) and analyzed by computer assisted image analysis techniques. It was found that both the volume fraction and number of the secondary carbides decreased during the hardening treatment process, while the mean diameter remained nearly unchanged, indicating critical effects of Ostwald ripening on the final carbide size distribution. Historically, however, studies on the heat treatment of this martensitic stainless steel focused mainly on secondary carbide dissolution, while little attention is paid to carbide coarsening. To better understand and ultimately provide a tool for the simulation of the concurrent occurrence of carbide dissolution and coarsening, mathematical carbide dissolution and coarsening model was developed incorporating a metallurgical kinetic theories of dissolution and Ostwald ripening. This was justified since most previous models were developed to predict only the mean carbide diameter and as such does not address the change in secondary carbide size distributions caused by concurrent dissolution and coarsening. Comparison of simulated distributions with those determined experimentally indicates that both dissolution and coarsening indeed occur concurrently during the hardening process. It was found that during austenitization the average radius of carbide particles increases quickly as small carbide particles dissolve in the austenite, but increases only slowly once small particles have disappeared. The nearly constant carbide radius maintained after the disappearance of small particles reflects comparable rates of carbide dissolution and coarsening. The cumulative amount of carbide dissolution increases while the average radius remains nearly constant. 3 ACKNOWLEDGEMENTS In this long and arduous journey, numerous individuals provided endless support and contributed tremendously to the realization of this work. First of all, I would like to express my sincere thanks to my academic/research advisor Professor Teiichi Ando of Northeastern University not only for his giving me a deep knowledge of fundamental metallurgy and a stronger focus on the objectives of this research, but also for his passionate encouragement, his vision and confidence on me for completing this work. His inspiration was crucial for me to fulfill the entire requirement for this Ph.D. at Northeastern University. I would also like to thank Professor Peter Wong of Tuft University and Professor Yung Joon Jung of Northeastern University for reviewing my work as the committee members as well as for openly sharing a wealth of their knowledge and selflessly giving me their precious time and insightful guidance. I would also like to thank many of my supporters, and my managers at The Proctor and Gamble Company for allowing me to pursue this endeavor. Special appreciation goes to Vivian Song for her assistance on MATLAB programming. My thanks also go to my fellow classmates, students and friends in the Advanced Materials Laboratory at Northeastern University in the past years for their assistance, support and friendship. I am very fortunate to be part of the team with Hui Lu, Hiroku Fukuda, Rajesh Ranganathan, Ibrahim Emre Gundus and Vivian Song. 4 Exceptional thanks also go to my parents, Jingtao Xu, and Guifeng Han, my husband, Mark, and my daughter, Stephanie, for their understanding, support, and sacrifice during the entire course of this work. 5 Table of Contents 1. Introduction ............................................................................................................................. 15 1.1 The history of stainless steel cutting instruments ............................................................ 16 1.2 The processes of making high quality cutting instruments using martensitic stainless steel 20 1.3 Research objectives and strategies ................................................................................... 22 2. Theories ................................................................................................................................... 23 2.1 Physical metallurgy of 13%Cr martensitic stainless steels ............................................. 24 2.2 Hardenability of martensitic stainless steels .................................................................... 32 2.2.1 Austenitization ............................................................................................................ 32 2.2.2 Quenching and sub-zero quenching .......................................................................... 34 2.2.3 Precipitation hardening and tempering ...................................................................... 37 2.3 Kinetics of hardening heat treatment ............................................................................... 39 2.3.1 Diffusion controlled secondary carbide dissolution .................................................. 39 2.3.2 Particle coarsening: Ostwald ripening ....................................................................... 43 3. Experimental Procedures ........................................................................................................ 45 3.1 Material characterization .................................................................................................. 46 3.1.1 Chemical composition of the material of interest ..................................................... 46 3.1.2 Surface characterization by Scanning electron microscopy (SEM) ......................... 46 3.1.3 Phase analysis by X-ray diffraction (XRD) ............................................................... 46 3.1.4 Hardness by Vicker’s hardness tester ........................................................................ 47 3.1.5 Determination of Retained austenite ......................................................................... 47 6 3.2 Hardening heat treatment and heat treatment equipment ................................................ 49 3.2.1 Austenitization ............................................................................................................ 49 3.2.2 Quenching ................................................................................................................... 50 3.2.3 Sub-zero quenching .................................................................................................... 50 3.2.4 Tempering or precipitation hardening Furnace ......................................................... 50 3.3 Design of hardening experiment ...................................................................................... 51 3.4 Secondary carbide Characterization ................................................................................ 53 3.4.1 Metallography ............................................................................................................ 54 3.4.2 SEM micrograph ........................................................................................................ 55 3.4.3 Quantitative image analysis ....................................................................................... 57 4. Experimental results and discussions ..................................................................................... 66 4.1 Material characterization
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